This invention relates to an improvement in rotary pressure leaf filters, and more particularly to apparatus for removing the filter cake which accumulates on the outer surfaces of filter leaves during the filtration process.
Although the apparatus of this invention is useful in separating many different types of solids suspended in various liquids, it is ideally suited for removing ash and mineral residue solids from liquid produced during the solvent refining of coal.
It has been known for many years that both liquid and solid solvent-refined coal can be made from a wide range of high-sulfur coals. In a typical process for making solvent-refined coal, raw coal is crushed and mixed with a creosote-like solvent. The mixture is pressurized, and hydrogen gas is added. The mixture is heated to convert most of the carbon and hydrocarbons in the coal to liquid form at the temperature and pressure of the process. Insoluble impurities, such as mineral residue and ash-forming impurities, remain suspended in solid form.
The industry has long sought an economical way to separate the solid impurities from the liquefied coal. For example, experts in the industry thought that it would not be practical to effect the separation in a filter which uses vertical filter leaves mounted to rotate about a horizontal axis, because it was believed that the filter cake would dislodge too easily from the filter leaf (septum) with slight pressure fluctuations across the filter medium inherent in most filtering operations. Consequently, the industry tried to use horizontal leaf filters, but they proved to be impractical, because the overhead motor and drive assembly interferes with the filter screen maintenance and inspection, and because they require inordinate overhead clearance for assembly and disassembly. Moreover, the horizontal screens crease and tear in coal liquefaction service. Another objection is that the horizontal leaves require precoating with a filter aid, such as diatomaceous earth or the like. The horizontal screens also become quickly blinded with the fine solids, because there is no effective method of cleaning them. For example, the horizontal leaves are rotated at high rpm to create centrifugal force to sling cake off leaves, but this has been unsatisfactory. Accordingly, the filter cycle of the horizontal leaf-type filter is relatively short, requiring frequent shutdown for clean-up and precoating of the filter leaves.
Rotary drum-type filters were also considered, but they required an unacceptable amount of filter aid medium.
Some prior art rotary pressure leaf filters have used fixed nozzles to direct a jet of sluicing liquid against the cake collected on the surface of the filter leaf. This has proved ineffective in removing filter cake collected during the filtering of solvent-refined coal, especially when no precoating filter aid is used.
U.S. Pat. No. 3,397,783 to Pearce discloses a nozzle mounted to move along an arcuate path in a plane perpendicular to the surface of a filter and direct a jet of sluicing liquid against the filter surface. Such apparatus is inefficient for removing difficult materials, such as the solids remaining in liquefied coal, because the angle of the jet relative to the surface, and the distance of the jet from the surface, changes as the nozzle moves. Thus, the nozzle is at the optimum angle and distance for only a brief portion of the cycle.
The filtering apparatus of this invention permits rapid and efficient filtration of the liquefied coal fraction to separate it from the solids originally present in the processed coal, because this invention makes possible efficient in-place cleaning of filter cake from filter leaves, even when no filter aid is used. The cleaning is accomplished quickly and thoroughly so that the cleaned filter leaf is restored to a condition virtually as good as new after each cleaning operation.